Component: Infiltration trenches

Description

Infiltration trenches are shallow excavations with rubble or stone that create temporary subsurface storage of stormwater runoff, thereby enhancing the natural capacity of the ground to store and drain water. Infiltration trenches allow water to exfiltrate into the surrounding soils from the bottom and sides of the trench.

Ideally they should receive lateral inflow from an adjacent impermeable surface, but point source inflows may be acceptable.

Advantages & disadvantages

Advantages

Disadvantages

Infiltration can significantly reduce both runoff rates and volumes

Infiltration provides a significant reduction in the pollutant load discharged to receiving body

Can be incorporated easily into site landscaping and fits well beside roads.

High clogging potential without effective pre-treatment – not for sites with fine particled soils (clay/silts) in upstream catchment

Build-up of pollution difficult to see

High historic failure rate due to poor maintenance, wrong siting or high debris input

Limited to relatively small catchments

Where component can be used

Residential: Yes

Commercial/industrial: Yes

High density: Yes

Retrofit: Yes

Contaminated sites: No

Sites above vulnerable groundwater: No

Performance

Peak flow reduction: Medium

Volume reduction: High

Water quality treatment: High

Amenity potential: Low

Ecology potential: Low

Quantity

Infiltration techniques:

provide storage for runoff in an underground chamber, lined with a porous membrane and filled with coarse crushed rock.

enhance the natural ability of the soil to drain the water. They do this by providing a large surface area in contact with the surrounding soil, through which the water can pass.

The amount of water that can be disposed of by an infiltration trench within a specified time depends mainly on the infiltration potential of the surrounding soil. The size of the device and the bulk density of any fill material will govern storage capacity.

Quality

Runoff is treated in different ways in an infiltration trench. These include:

physical filtration to remove solids

adsorption onto the material in the trench

biochemical reactions involving micro-organisms growing on the fill or in the soil.

The level of treatment depends on the size of the media and the length of the flow path through the system, which controls the time it takes the runoff to pass into the surrounding soil. Pre-treatment may be required before polluted runoff is allowed into an infiltration trench.

Amenity

Infiltration trenches are easy to integrate into a site. They are ideal for use around playing fields, recreational areas or public open space. They increase soil moisture content and help to recharge groundwater, thereby mitigating problems of low river flows.